Cystathionine β-synthase (CBS) plays an essential role in homocysteine metabolism in eukaryotes (Mudd et al., 2001, in The Metabolic and Molecular, Bases of Inherited Disease, 8 Ed., pp. 2007-2056, McGraw-Hill, New York). The CBS enzyme catalyzes a pyridoxal 5′-phosphate (PLP)-dependent condensation of serine and homocysteine to form cystathionine, which is then used to produce cysteine by another PLP-dependent enzyme, cystathionine γ-lyase. In mammalian cells that possess the transsulfuration pathway, CBS occupies a key regulatory position between the remethylation of Hcy to methionine or its alternative use in the biosynthesis of cysteine. The relative flux between these two competing pathways is roughly equal and is controlled by intracellular S-adenosylmethionine (AdoMet) concentrations (Finkelstein and Martin, 1984, J. Biol. Chem. 259(15), 9508-13). AdoMet activates the mammalian CBS enzyme by as much as 5-fold with an apparent dissociation constant of 15 μM (Finkelstein et al., 1975, Biochem. Biophys. Res. Commun. 66, 81-87; Roper et al., 1992, Arch. Biochem. Biophys. 298, 514-521; Kozich et al., 1992, Hum. Mutation 1, 113-123). Conversely, the same compound acts as an allosteric inhibitor of homocysteine remethylation by inhibiting 5,10-methylenetetrahydrofolate reductase (Kutzbach et al., 1967, Biochim. Biophys. Acta 139, 217-220; Kutzbach et al., 1971, Biochim. Biophys. Acta 250, 459-477) and betaine-homocysteine methyltransferase (Finkelstein et al., 1984, Biochem Biophys Res Commun 118(1), 14-9). Deficiency of CBS is the most common cause of inherited homocystinuria, a serious life threatening disease that results in severely elevated homocysteine levels in plasma, tissues and urine. Symptoms include dislocated optic lenses, skeletal disorders, mental retardation and premature arteriosclerosis and thrombosis (Mudd et al., 2001, supra).
Human CBS is a member of a large family of PLP-dependent enzymes that operate almost exclusively in the metabolism of amino acids. Members of this family are of multiple evolutionary origins (Salzmann et al., 2000, Biochem. Biophys. Res. Commun. 270(2), 576-80), but can be classified into four distinct families depending on their folds: the large α family, the β family, the D-alanine aminotransferase family, and the alanine racemase family (Alexander et al., 1994, Eur. J. Biochem. 219(3), 953-60). CBS belongs to the β family of PLP-dependent enzymes, members of which catalyze replacement and elimination reactions at Cβ.
While the catalytic cores of cysteine synthases (CS) and CBS enzymes exhibit high levels of homology, the N- and the C-terminal non-catalytic regions of these proteins show virtually no similarity. Human CBS (represented herein by SEQ ID NO:2) contains an N-terminal region of ˜70 amino acid (FIG. 1), which accommodates the heme prosthetic group (Meier et al., 2001, Embo J. 20(15), 3910-6). The function of this ligand is unknown but a number of studies indicate it may play either a regulatory or structural role (Taoka et al., 1998, J. Biol. Chem. 273, 25179-25184; Taoka and Banerjee, 2001, J Inorg. Biochem. 87(4), 245-51; Kery et al., 1994, 1 Biol. Chem. 269, 25283-25288; Kery, 1995, Arch. Biochem. Biophys. 316, 24-29). The observation that both yeast (Jhee et al., 2000, J. Biol. Chem. 275(16), 11541-4; Maclean et al., 2000, J. Inorg. Biochem. 81(3), 161-71; Jhee et al., 2000, Biochemistry 39(34), 10548-56) and Trypanosoma cruzi (Nozaki et al., 2001, J. Biol. Chem. 276(9), 6516-23) CBS lack heme indicates that it is not directly involved in catalysis (Maclean et al., 2000, supra; Jhee et al., 2000, Biochemistry, supra).
The C-terminal regulatory domain of human CBS consists of ˜140 amino acid residues (Kery et al., 1998, Arch. Biochem. Biophys. 355, 222-232). This region is required for tetramerization of the human enzyme and AdoMet activation (Kery et al., 1998, ibid.). The C-terminal regulatory region also encompasses the previously defined “CBS domain” (Bateman, 1997 Trends Biochem. Sci. 22, 12-13). This hydrophobic sequence (CBS1), spanning amino acid residues 415-468 of SEQ ID NO:2, is conserved in a wide range of otherwise unrelated proteins. Its function remains unknown, although the sharp transition of thermally induced CBS activation and the observation that mutations in this domain can constitutively activate the enzyme indicates that it plays a role in the autoinhibitory function of the C-terminal region (Janosik et al., 2001, Biochemistry 40(35), 10625-33; Shan et al., 2001, Hum. Mol. Genet 10(6), 635-643). Based on sequence similarity with another CBS domain containing protein, inosine 5′-monophosphate dehydrogenase (IMPDH) from Streptomyces pyogenes, a second, less conserved CBS domain (CBS2) has recently been identified between amino acid residues 486 to 543 of SEQ ID NO:2 in the C-terminal regulatory region of human CBS (FIG. 1, Shan et al., ibid.). Two well conserved CBS domains are also present in the C-terminal region of the yeast CBS, which is of approximately the same length as the human enzyme (FIG. 1). The yeast enzyme functions as a tetramer, but is not activated by AdoMet (Jhee et al., 2000, J. Biol. Chem. 275(16), 11541-4). CBS from T. cruzi, which is also unresponsive to AdoMet, lacks the typical CBS C-terminal region and exists predominantly as a tetramer. This observation has lead to speculation that CBS tetramerization is not exclusively a function of the C-terminal region (Nozaki et al., 2001, J. Biol. Chem. 276(9), 6516-23).
All of the CS enzymes lack both the N-terminal heme binding domain, and the C-terminal regulatory region (FIG. 1). These enzymes function as dimers, do not bind heme and are not activated by AdoMet (Byrne et al., 1988, J. Bacteriol. 170(7), 3150-7; Rolland et al., 1993, Arch. Biochem. Biophys. 300(1), 213-22).
Structure/function analyses of products derived from limited trypsinolysis of human CBS provided some initial insight into the domain architecture of this protein (Kery et al., 1998, supra). It was determined that the N-terminal 39 amino acid region does not play a significant role in the native structure of fully-folded CBS as removal of this region by partial tryptic cleavage does not affect AdoMet, PLP, heme binding, or tetramer formation (Kery et al., 1998, ibid.). Further proteolysis leads to the removal of the entire C-terminal regulatory region, yielding a proteolytically resistant core, consisting of amino acid residues 40-413 of SEQ ID NO:2. The removal of the C-terminal domain causes the enzyme to dissociate from tetramers to dimers. This change in oligomeric status of the enzyme is accompanied by an increase in tryptophan fluorescence, possibly caused by exposing a tryptophan cluster at positions 408-410 of SEQ ID NO:2. The truncated protein showed no change in both its UV and visible absorption spectra indicating that it maintains the structural features of full-length CBS and is unaffected in its ability to bind both PLP and heme (Kery et al., 1998, ibid.). The active core forms dimers and is about two to three-fold more active than the full-length tetramer, but cannot be further activated by AdoMet (Kery et al., 1998, ibid.).
Apart from AdoMet, several other modes of CBS activation have been reported. These include partial thermal denaturation (Janosik et al., 2001, supra), limited proteolysis (Kery et al., 1998, supra) and the presence of certain C-terminal mutations (Janosik et al., 2001, supra; Shan et al., 2001, supra). A possible common CBS activation mechanism has been proposed whereby the C-terminal region of CBS acts an autoinhibitory domain and that certain mutations, binding of AdoMet, limited trypsinolysis or partial thermal denaturation all serve to displace this domain from its zone of inhibition (Janosik et al., 2001, supra; Shan et al., 2001, supra).
A recombinant human CBS enzyme similar to the above-described “proteolytically resistant core” (i.e., 40-413 of SEQ ID NO:2) has recently been expressed in E. coli and purified to homogeneity (Janosik et al., 2001, Acta Crystallogr. D Biol. Crystallogr. 57(Pt 2), 289-291). This truncated enzyme, comprising amino acid residues 1-413 of SEQ ID NO:2, has been crystallized and its X-ray structure determined (Meier et al., 2001, supra). The crystals contained three dimers per asymmetric unit and each dimer contained one heme and one PLP per subunit. It was observed that the heme-binding region of the enzyme is almost completely disordered; the only exception is a short 310 helix formed by amino acid residues 60-62 of SEQ ID NO:2. Two N-terminal residues, Cys52 and His65 were identified as thiolate and histidine ligands to the heme. The heme resides in a small hydrophobic pocket at the outer end of each dimer, distant from the PLP cofactor, which is deeply buried in the active site and accessible only via a narrow channel (Meier et al., 2001, supra). The finding that the heme is relatively distant from the PLP and the fact that the heme iron is ligated from both sides by the amino acid residues provided evidence against its direct catalytic involvement (Meier et al., 2001, supra). However, the function of the heme group was still unknown at the time of the present invention.
U.S. Pat. No. 5,523,225 to Kraus, incorporated herein by reference in its entirety, describes the purified and isolated DNA for human cystathionine β-synthase (CBS), as well as restriction fragment length polymorphisms (RFLP) of the CBS gene, standard recombinant vectors comprising such DNA, recombinant host cells that express such DNA, and the protein encoded by the DNA. In this patent, conventional vectors were used to clone and express CBS.
U.S. Pat. No. 5,635,375 to Kraus, incorporated herein by reference in its entirety, describes a method of increasing the yield and heme saturation of cystathionine β-synthase produced by recombinant microorganisms. The method includes conventional expression of recombinant CBS fusion proteins in microorganisms (e.g., conventional expression vectors, production microorganisms and conditions were used), but with the incorporation of a heme precursor, such as δ-aminolevulinate, into the culture medium during the growth of the recombinant microorganisms. The inclusion of the heme precursor resulted in significantly improved CBS activity, yield of the enzyme and heme saturation of the enzyme.
U.S. Pat. No. 5,656,425 to Kraus, incorporated herein by reference in its entirety, describes a rapid screening process for detecting, localizing and expressing pathogenic mutations in the cystathionine β-synthase gene of a patient. The process includes the production of hybrid cDNAs of CBS DNA wherein subregions from the patient cDNA are expressed in the context of an otherwise wild-type CBS construct. The expression products of the hybrids are evaluated for decreased enzyme activity as a marker for pathogenic mutations with the patient cDNA.
To allow for effective and efficient purification of a recombinantly produced protein, it is conventional in the art to express the desired recombinant protein as part of a fusion protein, wherein the fusion partner is typically a protein that can: enhance a protein's stability, provide other desirable biological activity, and/or assist with the purification of a protein (e.g., by affinity chromatography). Fusion partners can be joined to amino and/or carboxyl termini of the recombinant protein to be produced, usually via a linker region to allow for the proper folding of the proteins in the fusion, and are typically susceptible to cleavage by a protease in order to enable straight-forward recovery of the desired recombinant protein. Cleavage of the fusion partner from the desired protein typically results in an extension of a few or several amino acid residues at the N- or C-terminal portion of the desired recombinant protein (depending on where the fusion partner is linked) which are heterologous to the recombinant protein sequence.
With regard to the CBS protein, which is routinely produced as a recombinant fusion protein, all of the publicly described recombinant GSH-CBS proteins described prior to the present invention have included a variable length (e.g., 12-23) of additional non-CBS amino acid residues at the amino terminus of the protein. This is a conventional result in the art, and it has not been discussed as an issue with regard to the production and use of the CBS protein. One of skill in the art can readily produce and purify an apparently functional CBS protein by conventional recombinant expression techniques even with the N-terminal extension artifact of the recombinant expression process. Moreover, as discussed above, previous studies have shown that the N-terminal 39 amino acid region does not play a significant role in the native structure of the fully-folded CBS, as the tryptic cleavage of this domain from the wild type enzyme does not affect AdoMet, PLP, heme binding, or tetramer formation (Kery et al., 1998, Arch. Biochem. Biophys. 355, 222-232). However, the present inventors, without being bound by theory, believe that the addition of non-human, non-CBS amino acid residues at the N-terminus of the CBS protein alters the properties of the enzyme. In addition, human CBS is a desirable therapeutic reagent, but the presence of non-human, non-CBS residues at the N-terminus of the recombinant CBS protein may have serious consequences for therapeutic applications, since these residues may elicit the formation of antibodies against the recombinant protein in human patients. Finally, while CBS is an attractive therapeutic molecule, there may be risks associated with administering the full-length protein or a nucleic acid encoding the same to a patient.
Therefore, there is a need in the art for an improved method to produce recombinant cystathionine β-synthase, including isoforms (variants) of the enzyme, that are effective and safe for use in human therapeutic applications.